1. Field of the Invention
This invention relates to a magnetic head, and more particularly to a magnetic head having a novel construction which is capable of removing any noise produced by external magnetic field during recording or play-back.
2. Description of the Prior Art
Generally, in a magnetic head, magnetic fluxes are produced from a tape driving motor for slidably moving a tape along the sliding surface of the magnetic head during play-back of the tape and from the earth magnetism and other external equipments, and such magnetic fluxes pass through the reproduce core of the magnetic head to produce induced voltages in the reproduce winding coiled on the reproduce core, which induced voltages may provide noises which may in turn be mixed with the normal output information from the magnetic tape or similar recording medium, thus failing to provide correct reproduction of the recorded information.
For this reason, the magnetic head usually has its core accommodated within a shield case to isolate and protect the core from external magnetic fluxes. Structurally, however, the magnetic head must have its core gap portion brought into direct contact with the recording medium in order to pick up the information from the recording medium sufficiently well and therefore, the sliding surface of the core is exposed out of the shield case. Thus, the sliding surface of the core has been the greatest weak point in shielding the magnetic head.
What countermeasures have heretofore been adopted as the means for eliminating the above-noted drawbacks will now be described by reference to FIGS. 1 to 3 of the accompanying drawings. FIGS. 1(a) and (b) are perspective views of the two-track magnetic head cores according to the prior art; FIG. 2(a) illustrates the relationship between the external magnetic field and the reproduce coil in the magnetic head core of FIG. 1(a) as enclosed in a shield case; FIG. 2(b) is a graph of characteristic curve showing the magnitude of the induced noise voltage, produced in the magnetic head of FIG. 2(a), with respect to the directions of the external magnetic field; FIG. 3(a) illustrates the relationship between the external magnetic head and the reproduce coil of the two-track magnetic head core of FIG. 1(b) as enclosed in a shield case; and FIG. 3(b) is a graph of characteristic curve showing the magnitude of the induced noise voltage, produced in the magnetic head of FIG. 3(a), with respect to the directions of the external magnetic field.
The magnetic head shown in FIG. 1(a) is known as the balance-coiled magnetic head which includes cores 1 each comprising a pair of bilaterally symmetrical left and right core halves 1-1 and 1-2 abutting with each other, and winding 2 wound on the left and right core halves 1-1, 1-2 in the same direction and connected in series with each other. Reference will now be had to FIG. 2(a) to describe the relationship of an external magnetic field and the noise resulting therefrom when such cores 1 are enclosed in a surrounding shield case. The external magnetic flux entering the core 1 in the horizontal direction with respect to the tape sliding surface thereof and passing outwardly therethrough as indicated by chain line 4 is herein called Hx, and the external magnetic flux entering the core 1 in the vertical direction with respect to the tape sliding surface thereof and passing downwardly therethrough as indicated by solid lines 5 is herein called Hy. On the left and right core halves 1-1 and 1-2, coils 2-1 and 2-2 formed by a winding 2 are connected in series with each other and wound so as to produce electromotive forces of the opposite senses for magnetic fluxes of the same direction, so that the external magnetic flux Hy vertically entering the core 1 as indicated by solid lines 5 passes through the left and right core halves 1-1, 1-2 to produce the electromotive forces therein, but such electromotive forces produced in these core halves are offset due to the coils being formed on these core halves by the same winding and in the same direction of turn, thus producing no noise output. The external magnetic flux Hx horizontally entering the core as indicated by the chain line 4 passes through the two coils on the left and right core halves as shown, whereas the electromotive forces produced thereby are not offset but provide noise voltages. Usually, however, the shield case formed of a highly permeable material causes considerable part of the horizontal external flux to bypass, so that the amount of the horizontal magnetic flux Hx which passes through the coils is smaller than in the direction Hy and accordingly, the magnetic flux Hx usually contributes to production of noise voltages at a much lower rate than the vertical magnetic flux Hy. Therefore, from the viewpoint to removing the noise created in the output coil, elimination of the noise voltages resulting from the vertical magnetic flux would be externally effective and thus, the magnetic head of FIG. 2(a) is also highly useful to prevent production of noise voltages. In FIG. 2(b) which shows the characteristic curve of the induced noise voltage in the above-described magnetic head with respect to the directions of the external magnetic flux, the length of the segment from the origin 0 to any point on the curve represents the magnitude of the induced noise voltage in the direction of that magnetic flux. It will thus be seen that the noise voltage produced by the external magnetic flux of y-direction (vertical direction) is the smallest.
However, the magnetic head as shown in FIG. 1(a) is useful in preventing the production of noise, whereas it encounters the following drawbacks when it is desired to construct a multichannel head. Nowadays, from the needs for smaller size and yet improved recording density of magnetic heads, it is often attempted that adjacent ones of the cores forming multiple channels be spaced apart as closely as possible, whereas in the construction as shown in FIG. 1(a), such attempts are restricted by the adjacent coils and this forms a great barrier in making the heads smaller. As a method of mounting coils in the multichannel magnetic head with the spacing between adjacent channels reduced as much as possible, it has heretofore been practised, as shown in FIG. 1(b), to wind a coil 8 only on one of the core halves, 6, forming each channel head and to employ, as its partner core half, a core such as core 7 formed with a cutaway portion P which prevents the coil and adjacent core from contacting each other between adjacent channel cores and which receives therein the coiled portion of an adjacent core. This method is useful in that a number of cores can be accommodated within a limited space, but as already mentioned, it is much inferior in prevention of the induced noise to the magnetic head constructed as shown in FIG. 1(a). Reference will now be had to FIG. 3(a) to consider the influence of the induced noise voltages on the vertical magnetic flux Hy and the horizontal magnetic flux Hx when the magnetic head core 9 comprising the core halves 6 and 7 as shown in FIG. 1(b) is accommodated with a shield case 10. As shown in FIG. 3(a), an output coil 8 is wound only on one of a pair of core halves, so that the external magnetic flux Hy vertically entering the core (indicated by solid lines 5 as in FIG. 1(a) produces an induced voltage in the coil 8 and such induced voltage appears as a noise at the output while a noise is also produced by the horizontal external magnetic flux Hx (indicated by chain line 4 as in FIG. 2(a)).
Thus, neither of the magnetic heads shown in FIGS. 1(a) and (b) has satisfied the two desires, i.e. prevention of externally induced noise and realization of smaller head size but both of them have had their own merits and demerits.